1. Field of the Invention
The present invention relates to an optical amplifier utilizing an optical amplifier fiber, especially to a simplified optical amplifier which can, keep the output light of the optical amplifier fiber constant and stable and which can reduce the output level of the excitation light source when the input light is cut off to the fiber, without need to detect the optical output level at a pre-stage of the fiber to reduce the output light of the excitation light source for exciting the energy level of an erbium-doped fiber, but just by detecting the optical output level at a post-stage of the fiber.
The photonic network, which uses an optical signal as a carrier for communications, has spread widely while meeting the demand for communications mainly because:                (1) The optical signal is resistant to electric noises,        (2) A repeater is substantially unnecessary but for a specific transmission line including an optical submarine transmission line, because the optical transmission line is low in transmission loss,        (3) It is suitable for high-speed data transmission because the optical transmission line has a wide band-width which is required by an optical signal as a communication carrier, and        (4) Conversion between an optical signal and an electric signal is performed at a high speed thanks to the advent of an electric-to-optical converter (hereinafter called e/o converter) and an optical-to-electric converter (hereinafter called o/e converter).        
A high-speed data transmission rate for even larger communication capacity is required to cope with a rapid increase in the communication demand brought about by the recent development of the Internet. Also, a demand for introduction of the photonic network into the subscriber lines is ever increasing. To introduce the photonic network into the subscriber lines, a wide dynamic range is required to cope with the variation in the length of the subscriber lines. Also, a significant cost reduction of the photonic network itself is required to mitigate an economical burden on the subscribers. Accordingly, an expansion of the dynamic range of the optical receiver device in the photonic network and a reduction of the cost of the optical amplifier are especially important.
2. Description of the Related Art
FIG. 4 shows an optical receiver (part 1) in a photonic network, along with an optical sender and an optical transmission line. In FIG. 4, the reference 1 shows an optical sender comprised of an e/o converter 11 and an optical output amplifier 12, for receiving an electric data signal from the data sender and converting the signal into an optical signal to output the optical signal to an optical transmission line.
2 is an optical transmission line for transmitting an optical signal modulated by transmission data. A single-mode optical fiber is chiefly used for the optical transmission line recently. 3 shows the optical receiver comprised of an o/e converter 32 and an electric signal regenerator 33, for converting a received optical signal into an electric signal and for regenerating data from the optical signal which was modulated by the data.
In the o/e converter 32 of FIG. 4, the received optical signal is supplied to a photodiode and the optical signal is converted into an electric (current) signal. In this case, if a usual photodiode is used, it is difficult to obtain a satisfactory dynamic range from the restriction that optical-to-electric conversion efficiency is not sufficiently high. If an avalanche photodiode with an electron multiplier function is used for the o/e-converter 32, the dynamic range of the optical receiver device can be greatly increased since the gain for a received optical signal can be increased equivalently.
However, attempting to increase the dynamic range by using the avalanche photodiode will cause the following problems:
(1) Since the avalanche photodiode is comparatively slow in response speed, it can convert an optical signal to an electric signal faithfully for one-digit gigabit-order data rate; however, for two-digit gigabit-order data rate, the converted electric signal waveform is distorted, causing a degradation of error rate in the electric signal reproduction circuit 33.
(2) Since the avalanche photodiode generates a comparatively large noise in the electron multiplication process, the S/N ratio in the o/e-converter 32 is reduced, causing a degradation of the error rate in the electric signal regenerator 33.
FIG. 5 shows an optical receiver (part 2) in a photonic network, along with an optical sender and an optical transmission line. In FIG. 5, reference 1 shows an optical sender which receives an electric data signal from a data sender and converts the signal into an optical signal to send to the optical transmission line. The optical sender 1 is comprised of an e/o converter 11 and an optical output amplifier 12. 2 shows an optical transmission line for transmitting the optical signal which was modulated with data. The single mode optical fiber is used chiefly for the line recently.
3a shows an optical receiver for converting the received optical signal into an electric signal, regenerating data by which the optical signal was modulated and supplying the data to a data receiver. The optical receiver 3a is comprised of an optical pre-amplifier 31, o/e converter 32 and electric signal regenerator 33. In FIG. 5, since the optical pre-amplifier 31 is disposed at the pre-stage of the o/e converter 32, the dynamic range can be expanded roughly by the amount of the gain provided by the optical pre-amplifier 31.
FIG. 1 shows a fundamental circuit configuration of a conventional optical amplifier which is applied to the optical output amplifier in FIG. 4 and FIG. 5 and optical pre-amplifier in FIG. 5. The optical output amplifier in FIGS. 4 and 5 is arranged to obtain a sufficient optical output level for the optical sender 1. The optical pre-amplifier in FIG. 5 is arranged to expand the dynamic range for the optical receiver 3.
In FIG. 1, reference 31-1 is a coupler for branching the input light into a main signal light and an input monitor light. 31-2 is an optical amplifier fiber for amplifying the main signal light. 31-3 is an excitation laser diode for generating an excitation light to be supplied to the optical amplifier fiber 31-2. The excitation laser diode is taken here as an example to supply the excitation light to the optical amplifier fiber. However, the excitation light source is not necessarily restricted to the excitation laser diode, but it can be any device that can generate light of a wavelength suited to excite an optical amplifier fiber.
31-4 is an excitation laser diode (abbreviated to LD) driver for supplying a driving current to the excitation laser diode 31-3. 31-5 is a coupler for coupling the excitation light outputted by the excitation laser diode 31-3 with the optical amplifier fiber 31-2. The excitation light is supplied to the optical amplifier fiber 31-2 through the coupler 31-5 and causes the main signal light to be amplified. The amplified main signal light propagates toward the output side through the coupler 31-5.
31-6 is a coupler for branching the optical signal from the coupler 31-5 into an output light and an output monitor light. 31-7 is an optical filter for removing the excitation light mixed in the output monitor light. The optical filter 31-7 is shown in FIG. 1 as an optical low-pass filter, meaning a filter to pass the light of low frequency. This is because, for a usual optical amplifier fiber which has erbium ion doped, the wave length of the excitation light is about 1.48 microns and that of the main signal light is about 1.55 microns and thus, the low-pass optical filter is required to remove the excitation light.
31-8 shows an o/e converter for converting the optical signal (simply called so here, although, to be strict, it includes the spontaneous emission light which is generated by the optical amplification in the optical amplifier fiber) outputted by the optical filter 31-7 into an electric signal. 31-9 is a reference voltage source. 31-10a is an automatic level control (abbreviated to ALC) circuit for keeping the output light level constant by comparing the output of the o/e converter 31-8 with the constant output voltage of the reference voltage source 31-9. The automatic level controller 31-10a supplies an automatic level control signal to the excitation laser diode driver 31-4. The output light level of the optical amplifier in FIG. 1 is kept, constant by controlling the output current of the excitation laser diode driver 31-4 according to the automatic level control signal and by controlling the level of the excitation light of the excitation laser diode 31-3.
31-11 is an optical filter for removing the excitation light element included in the input monitor light. The optical filter 31-7 is illustrated here too, as an optical low pass filter. The reason is the same as the optical filter 31-11 is an optical low pass filter.
31-12 is an o/e converter for converting the optical signal outputted by the optical filter 31-11 into an electric signal. 31-13 is an input level monitor for monitoring the level of the electric signal outputted by the o/e converter 31-12 and when the level of the optical signal is less than a defined level, halting the excitation laser (diode driver 31-4 to shut down the excitation light and for generating finally an output halt signal for halting the amplification operation of the optical amplifier fiber 31-2.
Here, the reason why the amplification operation by the optical amplifier fiber 31-2 is halted when it is determined that the input light level is less than the defined level due to the output halt signal outputted by the input level monitor 31-13 and the input light is shut down, is: (1) to prevent the excitation laser diode 31-3 from deteriorating due to the fact that when the level of the input light is less than the defined level, the output power of the excitation laser diode 31-3 becomes too large because the automatic level controller 31-10a attempts to keep the level of the output light constant (2) to prevent the post-stage optical parts and devices from deteriorating due to an optical surge caused by the output power of the excitation laser diode 31-3 being too large when the level of the input light recovers from the once-dropped level. That is, since the thus-constructed circuit in FIG. 1 can obtain a constant-level output light and even when the input light is shut down, can realize a reliable optical amplifier, both the optical sender 1 in FIG. 4 and optical receiver 3 in FIG. 5 can be guaranteed in performance and reliability.
However, since the optical amplifier is equipped with the o/e converter 31-12 including a set of the coupler 31-1, optical filter 31-11 and photodiode in order to halt the optical amplification operation by the input level monitor 31-13 and with the o/e converter 31-8 including a set of the coupler 31-6, optical filter 31-7 and photodiode in order to perform the automatic level control, a problem arises that the cost of the optical amplifier increases. This is not only because the optical devices per se including the coupler, optical filter and photodiode are costly, but because the parts required for mounting those devices are costly and also the adjusting and fixing work are costly.
To reduce the cost, the forward-controlled optical amplifier and backward-controlled optical amplifier have been developed. FIG. 2 shows a fundamental circuit configuration of a conventional forward-controlled optical amplifier. In FIG. 2, reference 31-1 is a coupler for branching the input light into a main signal light and input monitor light. 31-2 is an optical amplifier fiber for amplifying the main signal light. 31-3 is an excitation laser diode for generating the excitation light to be supplied to the optical amplifier fiber 31-2. 31-4 is an excitation laser diode driver for supplying a driving current to the excitation laser diode 31-3. 31-5 is a coupler for supplying the excitation light outputted by the excitation laser diode 31-3 to the optical amplifier fiber 31-2. 31-11 is an optical filter for removing the excitation light component mixed with the input monitor light. The optical filter 31-11 is shown here as a low-pass filter for the same reasons as already described.
31-12 is an o/e converter for converting an optical signal outputted by the optical filter 31-11 into an electric signal. 31-9 is a reference voltage source. 31-10a is an automatic level controller for keeping the output light at a constant level by comparing the output of the o/e converter 31-12 with the constant output voltage of the reference voltage source 31-9. The automatic level controller 31-10a supplies an automatic level control signal to the excitation laser diode driver 31-4. The output light of the optical amplifier in FIG. 2 is kept constant by controlling the output current of the excitation laser diode driver 31-4 and the excitation light level of the excitation laser diode 31-3 according to the automatic level control signal.
31-13 is an input level monitor for monitoring the electric output signal voltage of the o/e converter 31-12 when the output light signal level is lower than a defined level, i.e., when the input light level is lower than a defined level, for halting the excitation laser diode driver 31-4 to shut down the excitation light and generating finally an output halt signal for halting the amplification operation of the optical amplifier fiber 31-2. The reason is the same as aforesaid why the amplification operation of the optical amplifier fiber 31-2 is halted by the output halt signal outputted by the input level monitor 31-13 when the input light level is loner than the defined level.
The circuit in FIG. 2 features that the automatic level controller 31-10a generates the automatic level control signal by comparing the signal to which the aforesaid input monitor light has been o/e-converted with the constant output voltage of the reference voltage source 31-9, controls the driving current of the excitation laser diode 31-3, keeps the optical amplifier output level constant and when the input light level becomes lower than a defined level, halts the optical amplification operation. That is, when the optical amplifier in FIG. 2 is used as the optical output amplifier in FIGS. 4 and 5, the optical signal level on the optical transmission line can be raised high enough. When the optical amplifier in FIG. 2 is used as the optical pre-amplifier in FIG. 5, the dynamic range of the optical receiver 3 can be expanded.
Thus, the optical amplifier output level can be kept constant without monitoring its output level and when the input light level becomes lower than a defined level, the optical amplification operation can be halted. Thus, the cost of the optical amplifier and optical sender/receiver can be reduced.
FIG. 3 shows a fundamental circuit configuration of a conventional backward-controlled optical amplifier. In FIG. 3, reference 31-2 is an optical amplifier fiber for amplifying the input light. 31-3 is an excitation laser diode for generating an excitation light to be supplied to the optical amplifier fiber 31-2. 31-4 is an excitation laser diode driver for supplying a driving current to the excitation laser diode 31-3. 31-5 is a coupler for coupling the excitation light of the excitation laser diode 31-3 to the optical amplifier fiber 31-2. The excitation light is then supplied to the optical amplifier fiber 31-2 through the coupler 31-5 to amplify the main signal light. The amplified main signal light propagates to the output side through the coupler 31-5.
31-6 is a coupler for branching an optical signal output from the coupler 31-5 into an output light and an output monitor light. 31-7 is an optical filter for removing the excitation light which is mixed in the output monitor light.
The optical filter 31-7 is shown here as a low-pass filter for the same reasons as already described.
31-8 is an o/e converter for converting a light signal output from the optical filter 31-7 into an electric signal. 31-9 is a reference voltage source. 31-10a is an automatic level controller for keeping the output light level constant by comparing the output of the o/e converter 31-8 with the constant output voltage of the reference voltage source 31-9. The automatic level control circuit 31-10a supplies an automatic level control signal to the excitation laser diode driver 31-4. The output light level of the optical amplifier in FIG. 3 is kept constant by controlling the output current of the excitation laser diode driver 31-4 and the level of the excitation light of the excitation laser diode 31-3 according to the automatic level control signal.
The circuit in FIG. 3 features that the automatic level controller 31-10a generates the automatic level control signal by comparing the signal to which the input monitor light was o/e-converted with the constant output voltage of the reference voltage source 31-9, controls the driving current of the excitation laser diode 31-3 and keeps the optical amplifier output level constant. That is, when the optical amplifier in FIG. 3 is used as the optical output amplifier in FIGS. 4 and 5, the optical signal level on the optical transmission line can be kept high enough. When the optical amplifier in FIG. 3 is used as the optical pre-amplifier in FIG. 5, the dynamic range of the optical receiver 3 can be expanded. Then, the optical amplifier output level can be kept constant without monitoring the level of the light input to the optical amplifier. Thus, the cost of the optical amplifier and optical sender/receiver can be reduced.
In the circuit in FIG. 2, ambient temperature changes and aging have most influence on the excitation light level with respect to the driving current of the excitation laser diode 31-3. With the circuit in FIG. 2, the driving current is supplied to the excitation laser diode 31-3 only from the excitation laser diode driver 31-4 which is controlled by the feed-forward-controlled automatic level controller 31-10a. However, the circuit is not constructed such that it compensates the level fluctuations caused by the temperature changes and aging of the excitation laser diode 31-3, that is, the excitation laser diode 31-3 is not provided with a feedback for automatic level control.
Therefore, the circuit can keep the optical sender output level high enough, expand the dynamic range of the optical receiver and reduce the cost of the amplifier and the optical sender/receiver. However, since the circuit cannot prevent the changes in the output current of the excitation laser diode caused by the temperature changes and aging, the stability of the photonic network cannot be secured.
To solve this, it is necessary to set the output voltage of the reference voltage source so as to cancel the temperature change and aging characteristics of the excitation laser diode. This is not impossible in principle, but is difficult, to realize and results in a cost increase. That is, a circuit containing a resistor having a special temperature characteristic can cancel the aforesaid temperature characteristics, but it can hardly cancel the aging characteristics.
Thus, it is necessary to control the driving current by using a central processor based on the measured data of the temperature characteristic and predicted data of the excitation laser diode aging characteristic. However, using the central processing unit per se causes a cost increase. Moreover, it is very difficult to obtain accurate predicted data of the excitation laser diode aging characteristic, causing a further cost increase.
In the circuit in FIG. 3, since the automatic level controller 31-10a is controlled by the signal into which the output monitor light is o/e-converted, the influence of the ambient temperature change and aging on the level of the excitation light of the excitation laser diode 31-3 can be removed. However, in the aforesaid backward control method by simply using the signal into which the output monitor light is o/e-converted, the reliability of the excitation laser diode 31-3 decreases because when the input light is cutoff, the level of the excitation light of the excitation laser diode 31-3 rises very high. Also, the method cannot protect the post-stage optical parts and circuits from damages caused by an optical surge. The optical surge occurs because the level of the excitation light of the excitation laser diode 31-3 rises too high when the once-cut-off input light recovers.